In particular, the disclosure is used with a tractor, an agricultural tractor, a mobile working machine for cultivating fields or similar. Tractors or agricultural tractors are used as universally applicable working machines, primarily in agriculture for working on farmland. Ploughing, as one of the most frequent applications, places high requirements on the drive train of the tractor and the accuracy of control of an electronic-hydraulic lifting mechanism control system, so that the ploughing work can only be carried out quickly and efficiently with a coordinated operation. Above all, with highly inhomogeneous and very wet ground, owing to a very high resistance force to ploughing there is the possibility that the tractor will get stuck and/or the engine will stall. In addition to positional control of the plough, traction force control can be provided that seeks to prevent stalling of the engine by measuring the traction force at the linkage of the lower link on the tractor side and regulating the force or the gradient thereof to a value that prevents stalling of the engine or causes a reduction of the slip of the pressure rollers. This is carried out in particular by (briefly) raising the plough and the resulting reduction of the resistance force of the plough.
With very wet ground however, it is also possible that the tractor can become bogged down without the engine stalling. This is primarily detected in the event of a great deal of slip of the drive wheels, which can no longer provide a traction force or cannot provide sufficient traction force.
In order to limit the slip to an acceptable extent and to prevent excessive spinning of the drive wheels, it is known to equip the system of the electronic hydraulic lifting mechanism control system with a radar sensor or an ultrasound sensor that measures the actual speed of the tractor based on the Doppler effect. As in addition the wheel revolution rate of the drive wheels is known, the (current) slip of the drive wheels can be concluded by comparing the actual speed with the (expected) speed that is determined based on the wheel revolution rate. An additional slip control loop that is superimposed on the position and traction force control can ensure the maximum acceptable slip by raising the plough and thereby prevent bogging down of the tractor. In addition, a fixed predetermined target slip value can be regulated in order to potentially achieve energy efficient operation, because the traction forces are increased by this means and the engine may be able to be better utilized.
Even if the approaches outlined for the operation of such tractors or agricultural tractors will already produce good results, the available systems are nevertheless sometimes too complex and hence too cost intensive, not sufficiently reliable and/or too inaccurate. In this respect, there is a need for improvements, in particular during the determination of the drive slip or the determination of the actual speed.